1. Field of the Invention
This invention relates generally to low current, integrated circuit (IC) oscillators, and more particularly to a high efficiency oscillator circuit especially suited for operation in electronic watches.
2. Description of Related Art
Integrated circuit oscillators for use in electronic watches will frequently include a complementary (P type and N type) transistor pair input stage connected to a quartz crystal which determines circuit oscillation frequency. In order to provide a suitable level of output driving signal, this input complementary transistor pair will normally be connected to an output complementary transistor pair which is switched on and off at the crystal frequency to provide an AC rectangular wave output signal. This signal is then divided in frequency by a desired amount and used for driving the display of an electronic watch. Such an oscillator circuit is described, for example, in our U.S. pat. No. 4,360,789, assigned to the present assignee.
Using the above type of watch oscillator circuit, it has been found that parasitic current leakage paths in the input stage of the oscillator, and caused for example by moisture, will upset the balance of current in this input stage. in the absence of such parasitic current leakage paths, the current flow in each transistor of the input transistor pair will be equal. However, when these leakage paths are present and when DC coupling is used between the input and output stages, this current imbalance in the input stage is reflected in the output stage where it can produce unacceptable distortion in the AC waveform of the output stage.
In order to isolate the complementary-pair output stage from the above-described current imbalance of the input stage, one approach has been to use AC coupling between the input and output stages, and such an approach is disclosed, for example, by Vittoz in U.S. Pat. No. 4,013,797. However, when AC coupling is used to overcome the above problem of current imbalance and asymmetry in the input stage, a separate bias current generator is required for the output stage. This bias current generator produces a constant and undesirable DC power drain on the oscillator circuit, and this power drain is aggravated by the inability to provide suitably highvalued resistors in the bias current generator portion of the integrated circuit oscillator.